1. Field of the Invention
The present invention relates to a tandem type brake booster for use in the brake system of a vehicle such as an automobile, and particularly relates to a tandem type brake booster in which a rear power piston and a diaphragm associated therewith are designed to be reliably and firmly fixed to the valve body of the booster.
2. Description of the Prior Art
Recently, there has been a tendency for the engine of an automobile to be so designed as to operate at a higher speed and to output a higher power, and, corresponding to this tendency, many automobiles are provided with a brake booster designed to boost the force applied to the brake pedal by utilizing, for example, negative pressure in the intake manifold of the engine and to output the boosted force to the master cylinder. In the case where a relatively higher output is required, there is employed a tandem type brake booster in which two power pistons are arranged to work in tandem.
The following are conventional well-known arrangements in a brake booster design for securing a power piston and a diaphragm associated thereto to the valve body of the booster.
Japanese Patent Publication (Kokoku) No. 23913/78 which corresponds to U.S. Pat. No. 3,897,718 discloses, in FIGS. 2 and 3, a booster in which the valve body is provided with a groove formed on the outer periphery thereof and extending circumferentially thereabut. The groove is designed to receive therein the inner circumference portion of a rear power piston and a bead portion formed adjacent to the inner circumference portion of a diaphragm. The groove is further designed to receive a retainer or an additional ring which fits in the groove behind the diaphragm to fix the rear power piston and diaphragm to the valve body.
The above arrangement for securing a power piston and a diaphragm to a valve body suffers, however, from the following problems. In the arrangement, the retainer or the additional ring is fitted in the same groove as the rear power piston and the diaphragm are fitted in. The retainer or the additional ring therefore becomes loose in the groove and it is further possible for it to come out of the groove when the diaphragm is excessively deformed due to the pressure acting thereon or elastic force of the diagram is reduced by virtue of deterioration thereof with age.
Japanese Utility Model Public Disclosure (Kokai) No. 116392/76 discloses another arrangement in which a power piston is formed integrally with the valve body and a diaphragm is secured to the valve body with a bead portion thereof being fastened between the valve body and a retainer held on the valve body by a stop key. In most boosters, the valve body is usually provided with a passage for selectively introducing atmospheric air and negative pressure into a variable pressure chamber. This passage cannot however be utilized for the aforementioned stop key. Therefore, a separate hole for the stop key must be formed in the valve body.